See #16

Fixes the compilation problems, I hope

Python part still needs to be fixed, almost there, compiles in clang again

Add some forceeval and type double where possible

Thought it would maybe help with compilation

But not clear why this changes anything...

The key point is to speed up compilation, and make it easier to develop, by using the packaged C++ interface more extensively

Fix missing inline

Added multipolar terms; doesn't compile on mac anymore

Appears to be working, additional testing required. Generalizing to handle G&V and G&T correlation integrals remains